Ferritic stainless steel sheet

ABSTRACT

Provided is a ferritic stainless steel sheet excellent in terms of corrosion resistance with which a decrease in the quantity of surface defects and an improvement in toughness are realized at the same time. 
     The ferritic stainless steel sheet has a chemical composition containing, by mass %, C: 0.020% or less, Si: 0.05% to 0.40%, Mn: 0.05% to 1.00%, P: 0.040% or less, S: 0.030% or less, Al: 0.001% to 0.15%, Cr: 20.0% to 23.0%, Ni: 0.01% to 0.80%, Cu: 0.30% to 0.80%, Ti: 0.10% to 0.50%, Nb: 0.010% to 0.150%, Zr: 0.005% to 0.150%, N: 0.020% or less, and the balance being Fe and inevitable impurities, in which relational expression (1) below is satisfied. 
       Zr≤Nb≤Ti  (1)
 
     (Here, each of Zr, Nb, and Ti in relational expression (1) denotes the content (mass %) of the corresponding chemical element.)

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2016/004336, filed Sep. 26, 2016, which claims priority to Japanese Patent Application No. 2015-192442, filed Sep. 30, 2015, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a ferritic stainless steel sheet having excellent corrosion resistance, only a small quantity of surface defects, and excellent toughness.

BACKGROUND OF THE INVENTION

A ferritic stainless steel sheet, which does not contain Ni in a large amount, is a material having a lower price and more excellent price stability than those of an austenitic stainless steel sheet. In addition, ferritic stainless steel sheets are used in various applications such as building materials, transport machine, home electrical appliances, and kitchen appliances, since they are excellent in terms of rust resistance.

The kind of ferritic stainless steel sheet which is used particularly in a harsh corrosive environment is a SUS443J1-type stainless steel sheet (JIS G 4305), which has excellent corrosion resistance equivalent to that of a SUS304-type stainless steel sheet (JIS G 4305, 18-mass %-Cr-8-mass %-Ni-based), which is an austenitic stainless, steel, as a result of containing 20.0 mass % to 23.0 mass % of Cr, 0.3 mass % to 0.8 mass % of Cu, and sufficient amounts of stabilizing chemical elements (Ti, Nb, and Zr).

The kind of SUS443J1-type stainless steel which is commonly used is SUS443J1-type stainless steel containing mainly Ti as a stabilizing chemical element. Such steel is excellent in terms of workability because texture growth is promoted as a result of containing Ti. Moreover, since such steel is sufficiently softened even in the case where cold-rolled-sheet annealing is performed at a lower temperature than that for steel containing Nb, it is possible to manufacture such steel by using a cold-rolled-sheet annealing and pickling line which is used for common steel, which results in high productivity. However, in the case of Ti-containing SUS443J1-type stainless steel, there may be a case where streaks (surface defects), which deteriorate aesthetic appearance, occur on the surface. It is known that the above-mentioned streaks are caused by coarse TiN which is formed on the surface when casting is performed. In addition, there is a problem of low toughness with Ti-containing SUS443J1-type stainless steel. This is because coarse TiN, which becomes a preferential starting point at which fracturing occurs, is formed.

Patent Literature 1 and Patent Literature 2 describe the prevention of surface defects and the improvement of toughness regarding Ti-containing ferritic stainless steel.

Patent Literature 1 discloses a method for manufacturing Ti-containing ferritic stainless steel having excellent roping resistance and good surface quality. In Patent Literature 1, the surface defect of a cold-rolled and annealed steel sheet is prevented by controlling the solidifying temperature of the steel, casting temperature, and TiN-precipitating temperature in the steel so that a specified relationship is satisfied in order to control the precipitation of TiN when the molten steel is cast.

Patent Literature 2 discloses a ferritic stainless steel sheet which has excellent toughness and good corrosion resistance and which is excellent in terms of productivity and economic efficiency and a method for manufacturing the steel sheet. In Patent Literature 2, the toughness of a hot-rolled and annealed steel sheet and the toughness of a cold-rolled and annealed steel sheet are improved by allowing nitrides in the steel to exist in the form of ZrN.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. Hei 1-118341

PTL 2: Japanese Unexamined Patent Application Publication No. 2011-214060

SUMMARY OF THE INVENTION

Nowadays, in response to the diversification of home electrical appliances, there is a demand for a ferritic stainless steel sheet with which a decrease in streaks on the surface and excellent toughness are both realized at the same time in addition to excellent corrosion resistance.

However, in the case of the method according to Patent Literature 1, since TiN is purposefully precipitated in order to obtain the effect of increasing the equiaxial crystal ratio of a slab, it is not possible to achieve a sufficient effect of improving toughness and reducing surface defects. In the case of the method according to Patent Literature 2 also, since it is not possible to sufficiently prevent the formation of TiN in steel, it is not possible to obtain a sufficient effect of improving toughness and reducing surface defects.

An object of aspects of the present invention is to provide a ferritic stainless steel sheet excellent in corrosion resistance in which a decrease in the quantity of surface defects and an improvement in toughness are realized at the same time and which is sufficiently softened even in the case where cold-rolled-sheet annealing is performed at a temperature equivalent to that for conventional Ti-containing SUS443J1-type stainless steel.

The present inventors, in response to the problems described above, conducted comprehensive investigations in order to realize a decrease in the quantity of surface defects and an improvement in toughness at the same time and, as a result, found that it is possible to improve the toughness of Ti-containing SUS443J1-type stainless steel by adding appropriate amounts of Zr and Nb in combination to Ti-containing SUS443J1-type stainless steel in order to change the precipitation form of TiN, which causes a deterioration in toughness, without an increase in cold-rolled-sheet annealing temperature. Moreover, it was found that, since it is possible to precipitate Ti-based inclusions in a finely dispersed form by this effect, it is possible to decrease the quantity of surface defects of a steel sheet caused by TiN.

Specifically, it was found that, by controlling the contents of the stabilizing chemical elements (Ti, Nb, and Zr) in the chemical composition of a SUS443J1-type ferritic stainless steel sheet so that the content of Ti, which is the main stabilizing chemical element, is 0.10 mass % to 0.50 mass %, the Nb content, which is equal to or less than the Ti content, is 0.010 mass % to 0.150 mass %, and the Zr content, which is equal to or less than the Nb content, is 0.005 mass % to 0.150 mass %, it is possible to allow sufficient softening to occur even when cold-rolled-sheet annealing is performed at a temperature equivalent to that for a case where a stabilizing chemical element is limited to Ti, and it is possible to realize a decrease in the quantity of surface defects and a high toughness at the same time. The mechanism of these is supposed to be as follows.

As a result of Nb and Zr being contained in combination in steel, since complex carbonitrides of Ti, Zr, and Nb ((Ti, Zr, Nb)(C,N)), whose particle size is smaller than that of TiN formed in ferritic stainless steel containing only Ti, are dispersedly precipitated, an improvement in toughness and a decrease in the quantity of surface defects are realized.

Aspects of the present invention are based on the findings described above, and the subject matter of aspects of the present invention is as follows.

[1] A ferritic stainless steel sheet having a chemical composition containing, by mass %, C: 0.020% or less, Si: 0.05% to 0.40%, Mn: 0.05% to 1.00%, P: 0.040% or less, S: 0.030% or less, Al: 0.001% to 0.15%, Cr: 20.0% to 23.0%, Ni: 0.01% to 0.80%, Cu: 0.30% to 0.80%, Ti: 0.10% to 0.50%, Nb: 0.010% to 0.150%, Zr: 0.005% to 0.150%, N: 0.020% or less, and the balance being Fe and inevitable impurities, in which relational expression (1) below is satisfied.

Zr≤Nb≤Ti  (1)

(Here, each of Zr, Nb, and Ti in relational expression (1) denotes the content (mass %) of the corresponding chemical element.)

[2] The ferritic stainless steel sheet according to item [1], the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from Co: 0.01% to 0.50%, Mo: 0.01% to 0.30%, and W: 0.01% to 0.50%.

[3] The ferritic stainless steel sheet according to item [1] or [2], the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from V: 0.01% to 0.50%, B: 0.0003% to 0.0030%, Mg: 0.0005% to 0.0100%, Ca: 0.0003% to 0.0030%, Y: 0.001% to 0.20%, and REM (rare-earth metal): 0.001% to 0.10%.

[4] The ferritic stainless steel sheet according to any one of items [1] to [3], the steel sheet having the chemical composition further containing, by mass %, one or both selected from Sn: 0.001% to 0.50% and Sb: 0.001% to 0.50%.

According to aspects of the present invention, it is possible to obtain a ferritic stainless steel sheet having excellent corrosion resistance, only a small quantity of surface defects, and excellent toughness.

In addition, since sufficient softening occurs by performing cold-rolled-sheet annealing at a temperature equivalent to that for a case where a stabilizing chemical element is limited to Ti, there is an improvement in the productivity of a ferritic stainless steel sheet.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the influence of the contents of Ti and Nb on toughness and the quantity of surface defects under the condition of Zr≤Nb.

FIG. 2 is a diagram illustrating the influence of the contents of Nb and Zr on toughness and the quantity of surface defects under the condition of Nb Ti.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereafter, the embodiments of the present invention will be described. Here, the present invention is not limited to the embodiments described below.

The chemical composition of the ferritic stainless steel sheet according to aspects of the present invention has a chemical composition containing, by mass %, C: 0.020% or less, Si: 0.05% to 0.40%, Mn: 0.05% to 1.00%, P: 0.040% or less, S: 0.030% or less, Al: 0.001% to 0.15%, Cr: 20.0% to 23.0%, Ni: 0.01% to 0.80%, Cu: 0.30% to 0.80%, Ti: 0.10% to 0.50%, Nb: 0.010% to 0.150%, Zr: 0.005% to 0.150%, N: 0.020% or less, and the balance being Fe and inevitable impurities, in which relational expression (1) below is satisfied.

Zr≤Nb≤Ti  (1)

Here, each of Zr, Nb, and Ti in relational expression (1) denotes the content (mass %) of the corresponding chemical element.

In addition, the chemical composition described above may further contain, by mass %, one, two, or all selected from Co: 0.01% to 0.50%, Mo: 0.01% to 0.30%, and W: 0.01% to 0.50%.

In addition, the chemical composition described above may further contain, by mass %, one, two, or more selected from V: 0.01% to 0.50%, B: 0.0003% to 0.0030%, Mg: 0.0005% to 0.0100%, Ca: 0.0003% to 0.0030%, Y: 0.001% to 0.20%, and REM (rare-earth metal): 0.001% to 0.10%.

In addition, the chemical composition described above may further contain, by mass %, one or both selected from Sn: 0.001% to 0.50% and Sb: 0.001% to 0.50%.

Hereafter, each of the constituent chemical elements will be described. “%” used when describing the content of a constituent chemical element means “mass %”, unless otherwise noted.

C: 0.020% or less

C is a chemical element which is effective for improving the strength of steel. Such an effect is obtained in the case where the C content is 0.001% or more. However, in the case where the C content is more than 0.020%, there is a significant deterioration in corrosion resistance and workability. Therefore, the C content is set to be 0.020% or less, preferably 0.015% or less, or more preferably 0.010% or less.

Si: 0.05% to 0.40%

Si is a chemical element which is effective as a deoxidizing agent. Such an effect is obtained in the case where, the Si content is 0.05% or more. However, in the case where the Si content is more than 0.40%, there is a deterioration in workability due to an increase in the hardness of steel. In addition, in the case where the Si content is more than 0.40%, since there is a decrease in the amount of scale formed on the upper surface of a slab, which has a lubrication effect when hot rolling is performed, there is an increase in the quantity of surface defects. Therefore, the Si content is limited to be in the range of 0.05% to 0.40%, or preferably 0.05% to 0.25%. It is more preferable that the lower limit of the Si content be 0.08% or more. It is more preferable that the upper limit of the Si content be 0.15% or less.

Mn: 0.05% to 1.00%

Mn has a deoxidizing function. Such an effect of Mn is obtained in the case where the Mn content is 0.05% or more. On the other hand, in the case where the Mn content is more than 1.00%, since the precipitation and coarsening of MnS are promoted, there is a deterioration in corrosion resistance. Therefore, the Mn content is limited to be in the range of 0.05% to 1.00%. It is preferable that the lower limit of the Mn content be 0.10% or more, or more preferably 0.15% or more. It is preferable that the upper limit of the Mn content be less than 0.30%, or more preferably 0.25% or less.

P: 0.040% or less

P is a chemical element which deteriorates corrosion resistance. In addition, there is a deterioration in hot workability as a result of P being segregated at grain boundaries. Therefore, it is desirable that the P content be as small as possible, and the P content is set to be 0.040% or less, or preferably 0.030% or less.

S: 0.030% or less

S combines with Mn to form a precipitate, that is, MnS. Since the interface between such MnS and the base metal of stainless steel becomes a starting point at which pitting corrosion occurs, there is a deterioration in corrosion resistance. Therefore, it is preferable that the S content be smaller, and the S content is set to be 0.030% or less, or preferably 0.020% or less.

Al: 0.001% to 0.15%

Al is a chemical element which is effective for deoxidation. Such an effect is obtained in the case where the Al content is 0.001% or more. On the other hand, in the case where the Al content is more than 0.15%, since there is a decrease in the amount of scale formed on the surface of a slab, which has a lubrication effect when hot rolling is performed, there is an increase in the quantity of surface defects. Therefore, the Al content is limited to be in the range of 0.001% to 0.15%. It is preferable that the lower limit of the Al content be 0.005% or more, or more preferably 0.01% or more. It is preferable that the upper limit of the Al content be 0.10% or less, or more preferably 0.05% or less.

Cr: 20.0% to 23.0%

Cr is a chemical element which improves corrosion resistance by forming a passive film on the surface. It is not possible to achieve sufficient corrosion resistance in the case where the Cr content is less than 20.0%. On the other hand, in the case where the Cr content is more than 23.0%, there is a tendency for toughness to deteriorate due to a phase and 475° C. embrittlement. Therefore, the Cr content is set to be 20.0% to 23.0%. It is preferable the lower limit of the Cr content be 20.5% or more. It is preferable that the upper limit of the Cr content be 22.0% or less, or more preferably 21.5% or less.

Ni: 0.01% to 0.80%

Ni is a chemical element which makes it possible to maintain a passive state even at a lower pH by inhibiting an anode reaction due to acid. That is, Ni improves corrosion resistance by markedly inhibiting the progress of corrosion in an active dissolution state as a result of increasing the effect of crevice corrosion resistance. Such an effect of Ni is obtained in the case where the Ni content is 0.01% or more. On the other hand, in the case where the Ni content is more than 0.80%, there is a deterioration in workability due to an increase in the hardness of steel. Therefore, the Ni content is limited to be 0.01% to 0.80%. It is preferable that the lower limit of the Ni content be 0.05% or more, or more preferably 0.10% or more. It is preferable that the upper limit of the Ni content be 0.40% or less, or more preferably 0.25% or less.

Cu: 0.30% to 0.80%

Cu is a chemical element which improves corrosion resistance by strengthening a passive film. On the other hand, in the case where the Cu content is excessively large, since c-Cu tends to be precipitated, there is a deterioration in corrosion resistance. Therefore, the Cu content is set to be 0.30% to 0.80%. It is preferable that the lower limit of the Cu content be 0.35% or more, or more preferably 0.40% or more. It is preferable that the upper limit of the Cu content be 0.60% or less, or more preferably 0.45% or less.

Ti: 0.10% to 0.50%

Ti is a chemical element which improves corrosion resistance by preventing sensitization due to Cr carbonitrides as a result of fixing C and N. However, TiN, which is formed as a result of containing Ti, causes a deterioration in toughness. In accordance with aspects of the present invention, the above-mentioned deterioration in toughness due to Ti is suppressed by the combination effect of Nb and Zr as described below. The effect of improving corrosion resistance through the use of Ti is obtained in the case where the Ti content is 0.10% or more. On the other hand, in the case where the Ti content is more than 0.50%, there is a deterioration in workability due to an increase in the hardness of a stainless steel sheet. In addition, in the case where the Ti content is more than 0.50%, since it is difficult to control the precipitation form of Ti-based inclusions even in the case where Nb and Zr are contained, there is a deterioration in surface quality. Therefore, the Ti content is set to be in the range of 0.10% to 0.50%. It is preferable that the lower limit of the Ti content be 0.15% or more, or more preferably 0.18% or more. It is preferable that the upper limit of the Ti content be 0.35% or less, or more preferably 0.26% or less.

Nb: 0.010% to 0.150%

Nb is, like Ti, a chemical element which improves corrosion resistance by preventing sensitization due to Cr carbonitrides as a result of fixing C and N. Moreover, Nb improves toughness and inhibits a surface defect from occurring by the combination effect with Zr described below. Such effects are obtained in the case where the Nb content is 0.010% or more. On the other hand, in the case where the Nb content is more than 0.150%, there is a deterioration in workability due to an increase in the hardness of a stainless steel sheet. In addition, in the case where the Nb content is more than 0.150%, since there is an increase in recrystallization temperature, there is a deterioration in manufacturability. Therefore, the Nb content is set to be in the range of 0.010% to 0.150%. It is preferable that the lower limit of the Nb content be 0.030% or more, or more preferably 0.070% or more. It is preferable that the upper limit of the Nb content be less than 0.100%, or more preferably 0.090% or less.

Zr: 0.005% to 0.150%

Zr is, like Ti, a chemical element which improves corrosion resistance by preventing sensitization due to Cr carbonitrides as a result of fixing C and N. Moreover, Zr improves toughness and inhibits a surface defect from occurring by the combination effect with Nb described below. It is necessary that the Zr content be 0.005% or more in order to obtained such effects. On the other hand, in the case where the Zr content is more than 0.150%, since Zr-based inclusions are precipitated on the surface, there is an increase in the quantity of surface defects. Therefore, the Zr content is limited to be in the range of 0.005% to 0.150%. It is preferable that the lower limit of the Zr content be 0.010% or more, or more preferably 0.030% or more. It is preferable that the upper limit of the Zr content be less than 0.100%, or more preferably 0.080% or less.

It was found that, in accordance with aspects of the present invention, by containing Nb and Zr in combination to SUS443J1-type stainless steel which contains only Ti as a stabilizing chemical element, it is possible to allow sufficient softening to occur by performing cold-rolled-sheet annealing even at a temperature equivalent to that for the case where a stabilizing chemical element is limited to Ti, and it is possible to realize a decrease in the quantity of surface defects and a high toughness at the same time. Specifically, it was found that, by controlling the contents of the stabilizing chemical elements (Ti, Nb, and Zr) in the chemical composition of a SUS443J1-type stainless steel so that the Ti content is 0.10% to 0.50%, the Nb content is 0.010% to 0.150%, and the Zr content is 0.005% to 0.150% under the condition expressed by relational expression (1) below, it is possible to allow sufficient softening to occur by performing cold-roiled-sheet annealing even at a temperature equivalent to that for the case where a stabilizing chemical element is limited to Ti, and it is possible to realize a decrease in the quantity of surface defects and a high toughness at the same time. The mechanism of these is supposed to be as follows.

It is considered that, as a result of Nb and Zr being contained in combination in steel, since complex carbonitrides of Ti, Zr, and Nb ((Ti, Zr, Nb)(C,N)), whose particle size is smaller than that of TiN formed in ferritic stainless steel containing only Ti, are dispersedly precipitated, an improvement in toughness and a decrease in the quantity of surface defects are realized. In order to form the above-mentioned complex carbonitrides ((Ti, Zr, Nb)(C,N)) in sufficient amounts, it is necessary that relational expression (1) below be satisfied.

Zr≤Nb≤Ti  (1)

Here, each of Zr, Nb, and Ti in relational expression (1) denotes the content (mass %) of the corresponding chemical element.

Regarding the relationship between Ti and Nb, it is preferable that the relational expression Ti≥1.5Nb, or more preferably Ti≥2Nb, be satisfied. Regarding the relationship between Nb and Zr, it is preferable that the relational expression Nb≥1.3Zr, or more preferably Nb≥1.5Zr, be satisfied.

N: 0.020% or less

N is a chemical element which is inevitably mixed in steel. However, in the case where the N content is more than 0.020%, there is a significant deterioration in corrosion resistance and workability. Therefore, the N content is set to be 0.020% or less, or preferably 0.015% or less.

The basic constituent chemical elements are described above, and the chemical elements described below may be appropriately added in addition to the basic constituent chemical elements in accordance with aspects of the present invention as described above.

Co: 0.01% to 0.50%

Co is a chemical element which improves the crevice corrosion resistance of stainless steel. Such an effect of Co is obtained in the case where the Co content is 0.01% or more. However, in the case where the Co content is more than 0.50%, such an effect of Co becomes saturated, and there is a deterioration in workability. Therefore, in the case where Co is contained, the Co content is set to be 0.01% to 0.50%. It is preferable that the lower limit of the Co content be 0.02% or more, or more preferably 0.03% or more. It is preferable that the upper limit of the Co content be 0.30% or less, or more preferably 0.10% or less.

Mo: 0.01% to 0.30%

Mo is effective for improving the crevice corrosion resistance of stainless steel. Such an effect is obtained in the case where the Mo content is 0.01% or more. However, in the case where the Mo content is more than 0.30%, such an effect of Mo becomes saturated, and there is a deterioration in toughness due to the formation of coarse intermetallic compounds. Therefore, in the case where Mo is added, the Mo content is set to be 0.01% to 0.30%. It is preferable that the lower limit of the Mo content be 0.02% or more, or more preferably 0.03% or more. It is preferable that the upper limit of the Mo content be 0.20% or less, or more preferably 0.10% or less.

W: 0.01% to 0.50%

W is a chemical element which improves the crevice corrosion resistance of stainless steel. Such an effect of W is obtained in the case where the W content is 0.01% or more. On the other hand, in the case where the W content is more than 0.50%, such an effect of W becomes saturated, and there is a deterioration in workability. Therefore, in the case where W is contained, the W content is set to be 0.01% to 0.50%. It is preferable that the lower limit of the W content be 0.02% or more, or more preferably 0.03% or more. It is preferable that the upper limit of the W content be 0.30% or less, or more preferably 0.10% or less.

V: 0.01% to 0.50%

V is a chemical element which improves the crevice corrosion resistance of stainless steel. Such an effect of V is obtained in the case where the V content is 0.01% or more. On the other hand, in the case where the V content is more than 0.50%, such an effect of V becomes saturated, and there is a deterioration in workability. Therefore, in the case where V is added, the V content is set to be 0.01% to 0.50%, preferably 0.01% to 0.30%, or more preferably 0.01% to 0.10%.

B: 0.0003% to 0.0030%

Since B is a chemical element which improves hot workability and secondary workability, it is effective to containing B to Ti-containing steel. Such an effect of B is obtained in the case where the B content is 0.0003% or more. On the other hand, in the case where the B content is more than 0.0030%, there is a deterioration in toughness. Therefore, in the case where B is contained, the B content is set to be in the range of 0.0003% to 0.0030%. It is preferable that the lower limit of the B content be 0.0015% or more. It is preferable that the upper limit of the B content be 0.0025% or less.

Mg: 0.0005% to 0.0100%

Mg functions as a deoxidizing agent along with Al by forming Mg oxides in molten steel. Such an effect of Mg is obtained in the case where the Mg content is 0.0005% or more. On the other hand, in the case where the Mg content is more than 0.0100%, there is a deterioration in manufacturability and a deterioration in the toughness of steel. Therefore, in the case where Mg is contained, the Mg content is limited to be in the range of 0.0005% to 0.0100%. It is preferable that the lower limit of the Mg content be 0.0010% or more. It is preferable that the upper limit of the Mg content be 0.0050% or less, or more preferably 0.0030% or less.

Ca: 0.0003% to 0.0030%

Ca is a chemical element which improves hot workability. Such an effect of Ca is obtained in the case where the Ca content is 0.0003% or more. On the other hand, in the case where the Ca content is more than 0.0030%, there is a deterioration in the toughness of steel, and there is a deterioration in corrosion resistance due to the precipitation of CaS. Therefore, in the case where Ca is added, the Ca content is limited to be in the range of 0.0003% to 0.0030%. It is preferable that the lower limit of the Ca content be 0.001% or more. It is preferable that the upper limit of the Ca content be 0.002% or less.

Y: 0.001% to 0.20%

Y is a chemical element which improves cleanliness by inhibiting a decrease in the viscosity of molten steel. Such an effect of Y is obtained in the case where the Y content is 0.001% or more. On the other hand, in the case where the Y content is more than 0.20%, such an effect of Y becomes saturated, and there is a deterioration in workability. Therefore, in the case where Y is added, the Y content is limited to be in the range of 0.001% to 0.20%, or preferably 0.001% to 0.10%.

REM (rare-earth metal): 0.001% to 0.10%

REM (rare-earth metal: one of the chemical elements having atomic numbers of 57 through 71 such as La, Ce, or Nd) is a chemical element which improves high-temperature oxidation resistance. Such an effect of REM is obtained in the case where the REM content is 0.001% or more. On the other hand, in the case where the REM content is more than 0.10%, such an effect of REM becomes saturated, and a surface defect occurs when hot rolling is performed. Therefore, in the case where REM is contained, the REM content is limited to be in the range of 0.001% to 0.10%. It is preferable the lower limit of the REM content be 0.005% or more. It is preferable that the upper limit of the REM content be 0.05% or less.

Sn: 0.001% to 0.50%

Sn is effective for improving ridging resistance by promoting the formation of a deformation zone when rolling is performed. Such an effect is obtained in the case where the Sn content is 0.001% or more. However, in the case where the Sn content is more than 0.50%, such an effect of Sn becomes saturated, and there is a deterioration in workability. Therefore, in the case where Sn is added, the Sn content is set to be 0.001% to 0.50%. It is preferable that the lower limit of the Sn content be 0.003% or more. It is preferable that the upper limit of the Sn content be 0.20% or less.

Sb: 0.001% to 0.50%

Sb is effective for improving ridging resistance by promoting the formation of a deformation zone when rolling is performed. Such an effect is obtained in the case where the Sb content is 0.001% or more. However, in the case where the Sb content is more than 0.50%, such an effect of Sb becomes saturated, and there is a deterioration in workability. Therefore, in the case where Sb is contained, the Sb content is set to be 0.001% to 0.50%. It is preferable that the lower limit of the Sb content be 0.003% or more. It is preferable that the upper limit of the Sb content be 0.20% or less.

The remainder which is other than the constituent chemical elements described above is Fe and inevitable impurities. Representative examples of the inevitable impurities described here include H, O (oxygen), Zn, Ga, Ge, As, Ag, In, Hf, Ta, Re, Os, Ir, Pt, Au, and Pb. Among these chemical elements, H and O (oxygen) may be contained in an amount of 0.05% or less. Other chemical elements may be contained in an amount of 0.3% or less.

Hereafter, a preferable method for manufacturing the ferritic stainless steel sheet according to aspects of the present invention will be described. Molten steel having the chemical composition described above is prepared by using a known method such as one which utilizes a converter, an electric furnace, or a vacuum melting furnace and made into a steel material (slab) by using a continuous casting method or an ingot casting-slabbing method. This steel material is heated to a temperature of 1000° C. to 1200° C. and then subjected to hot rolling so as to have a thickness of 2.0 mm to 5.0 mm under the condition of a finishing temperature of 700° C. to 1000° C. The hot-rolled steel sheet, which has been obtained as described above, is subjected to annealing at a temperature of 800° C. to 1100° C. followed by pickling, cold rolling, and cold-rolled-sheet annealing at a temperature of 700° C. to 1000° C. After cold-rolled-sheet annealing has been performed, pickling is performed in order to remove scale. The cold-rolled steel sheet from which scale has been removed may be subjected to skin pass rolling.

In addition, aspects of the present invention are effective not only for the above-mentioned cold-rolled sheet product but also for a hot-rolled sheet product.

Examples

After ferritic stainless steels having the chemical compositions given in Table 1 (Table 1-1 and Table 1-2 are combined to form Table 1), Table 2 (Table 2-1 and Table 2-2 are combined to form Table 2), and Table 3 (Table 3-1 and Table 3-2 are combined to form Table 3) had been made into steel ingots having a weight of 100 kg, the ingots were heated to a temperature of 1200° C. and subjected to hot rolling in order to obtain hot-rolled steel sheets having a thickness of 4.0 mm. Subsequently, the hot-rolled steel sheets were subjected to annealing at a temperature of 1100° C. followed by pickling which utilized a commonly used method and subjected to cold rolling so as to have a thickness of 2.0 mm followed by annealing at a temperature of 900° C. and pickling which utilized a commonly used method.

By determining the pitting potential (JIS G 0577) of the obtained cold-rolled and annealed steel sheet, corrosion resistance was evaluated. A case where the pitting potential was 290 mV (vs. SCE) or more was judged as “◯” (satisfactory), and a case where the pitting potential was less than 290 mV (vs. SCE) was judged as “▴” (unsatisfactory).

In addition, by performing a Charpy impact test on a test piece (JIS B 7722, V notch) which had been taken from the obtained cold-rolled and annealed steel sheet along the rolling direction, the toughness of the steel sheet was evaluated. A case where the Charpy impact value at 25° C. was 200 J/cm² or more was judged as “◯” (satisfactory), and a case where the Charpy impact value at 25° C. was less than 200 J/cm² was judged as “▴” (unsatisfactory).

Moreover, by observing the surface of the cold-rolled and annealed steel sheet in order to determine the density of streaks on the surface, the quantity of surface defects was evaluated. By preparing 10 steel sheets having each of the chemical compositions, and by counting the number of streaks having a length in the L-direction of more than 10 mm in a region having a width of 200 mm and a length of 200 mm on the center portion of the upper surface of each of the steel sheets, a case where the average of the counted numbers was 1 or less was judged as “◯” (satisfactory), and a case where the average of the counted numbers was more than 1 was judged as “▴” (unsatisfactory).

Moreover, it was evaluated whether sufficient softening occurred by performing annealing even at a temperature of 880° C. for 20 seconds on the cold-rolled steel sheet which had not yet been annealed. The evaluation was performed by comparing the hardness (a) of a steel sheet in the cold-rolled state or as cold-rolled, the hardness (b) of a steel sheet which had been subjected to annealing at a temperature of 880° C. for 20 seconds, and the hardness (c) of a steel sheet which had been subjected to annealing at a temperature of 1000° C. for 20 seconds as an index of a case where sufficient softening occurred. Three test pieces having a length of 15 mm and a width of 20 mm were taken from the cold-rolled steel sheet, and each of the test pieces for respectively determining b and c was subjected to the corresponding annealing. Subsequently, each of the three test pieces were cut into pieces having a length of 15 mm and a width of 10 mm. Then, the Vickers hardness determined in the cross section of the cut piece was used for the evaluation. As annealing progressed, the harness of the steel sheet changed from a to c. A case where 90% or more of such a softening process was completed through annealing at a temperature of 880° C. for 20 seconds, that is, a case where the relational expression c+0.1×(a−c)≥b was satisfied, was judged as “◯” (satisfactory), and other cases were judged as “▴” (unsatisfactory).

The obtained results are given in Tables 1, 2, and 3. In the case of the steels of the examples of the present invention, all the judgment results of the determined pitting potential, the Charpy impact value, the surface defect, and the softening temperature were “◯”, which means that these steels had good corrosion resistance and toughness, only a small quantity of surface defects, and no manufacturing problem.

Comparative example No. 34, whose Cr content was lower than the range according to aspects of the present invention, had poor corrosion resistance.

Comparative example No. 35, whose Cr content was higher than the range according to aspects of the present invention, had poor toughness.

Comparative example No. 36, whose Ni content was lower than the range according to aspects of the present invention, had poor corrosion resistance.

Comparative example No. 37, whose Ti content was lower than the range according to aspects of the present invention, had poor corrosion resistance.

Comparative example No. 38, whose Ti content was higher than the range according to aspects of the present invention, had poor toughness and a large quantity of surface defects.

Comparative example No. 39, whose Nb content was lower than the range according to aspects of the present invention, had poor toughness and a large quantity of surface defects.

Comparative example No. 40, whose Nb content was higher than the range according to aspects of the present invention, had poor manufacturability due to a high softening temperature.

Comparative example No. 41, whose Zr content was lower than the range according to aspects of the present invention, had poor toughness and a large quantity of surface defects.

Comparative example No. 42, whose Zr content was higher than the range according to aspects of the present invention, had a large quantity of surface defects.

Comparative example No. 57, whose contents of Nb and Zr were both lower than the range according to aspects of the present invention, had poor toughness and a large quantity of surface defects.

Comparative example No. 58, whose contents of Ti and Zr were both lower than the ranges according to aspects of the present invention, and whose contents of Al and Nb were both higher than the ranges according to aspects of the present invention, had poor toughness, a large quantity of surface defects, and poor manufacturability due to a high softening temperature.

Here, comparative example Nos. 43 through 54, 67, and 68 will be described hereafter with reference to FIG. 1 and FIG. 2.

TABLE 1-1 Test Chemical Composition (mass %) No. C Si Mn P S Al Cr Ni Cu Ti Nb Zr N Other Chemical Element Note 1 0.010 0.09 0.21 0.025 0.002 0.020 20.2 0.29 0.43 0.19 0.075 0.031 0.009 Example 2 0.012 0.12 0.17 0.021 0.002 0.026 21.1 0.30 0.44 0.24 0.071 0.034 0.007 Example 3 0.011 0.11 0.19 0.029 0.003 0.027 21.2 0.28 0.42 0.19 0.089 0.012 0.008 Example 4 0.012 0.10 0.22 0.027 0.002 0.035 22.7 0.18 0.41 0.20 0.088 0.011 0.013 Example 5 0.009 0.11 0.20 0.021 0.002 0.029 20.6 0.02 0.44 0.22 0.095 0.034 0.008 Example 6 0.008 0.12 0.16 0.028 0.002 0.037 20.6 0.21 0.44 0.21 0.076 0.021 0.008 Example 7 0.010 0.12 0.20 0.028 0.003 0.032 21.4 0.79 0.41 0.24 0.069 0.023 0.010 Example 8 0.010 0.09 0.16 0.028 0.003 0.024 20.9 0.27 0.41 0.11 0.055 0.036 0.013 Example 9 0.011 0.10 0.22 0.020 0.002 0.035 20.9 0.29 0.45 0.22 0.075 0.020 0.010 Example 10 0.007 0.10 0.16 0.027 0.001 0.031 20.7 0.23 0.42 0.48 0.079 0.023 0.010 Example 11 0.012 0.12 0.18 0.023 0.003 0.028 20.5 0.19 0.43 0.20 0.012 0.007 0.008 Example 12 0.011 0.12 0.19 0.030 0.001 0.026 20.9 0.13 0.40 0.21 0.077 0.040 0.013 Example 13 0.010 0.11 0.19 0.026 0.003 0.031 20.9 0.12 0.43 0.32 0.147 0.032 0.012 Example 14 0.011 0.11 0.18 0.026 0.002 0.033 21.2 0.24 0.42 0.24 0.094 0.006 0.013 Example 15 0.008 0.09 0.19 0.028 0.003 0.033 20.7 0.11 0.41 0.23 0.081 0.047 0.010 Example 16 0.012 0.08 0.17 0.030 0.002 0.036 21.4 0.18 0.43 0.40 0.148 0.146 0.012 Example 17 0.009 0.09 0.16 0.029 0.002 0.032 21.4 0.24 0.41 0.12 0.072 0.051 0.007 Example 18 0.013 0.08 0.21 0.021 0.002 0.024 20.9 0.25 0.45 0.17 0.092 0.065 0.008 Example 19 0.007 0.10 0.20 0.022 0.002 0.148 20.6 0.15 0.42 0.21 0.123 0.090 0.008 Example 20 0.008 0.08 0.17 0.027 0.002 0.026 21.3 0.21 0.45 0.11 0.099 0.092 0.008 Example 21 0.009 0.12 0.20 0.026 0.002 0.023 20.8 0.25 0.41 0.16 0.112 0.095 0.012 Example 22 0.009 0.11 0.15 0.026 0.003 0.037 21.1 0.26 0.43 0.21 0.147 0.118 0.012 Example 23 0.009 0.10 0.16 0.029 0.002 0.034 20.7 0.18 0.43 0.21 0.061 0.014 0.007 Co: 0.07, Mo: 0.05, W: 0.08 Example 24 0.007 0.10 0.20 0.021 0.003 0.029 20.7 0.14 0.45 0.19 0.070 0.011 0.010 V: 0.07, Ca: 0.0012, La: 0.03 Example 25 0.012 0.09 0.20 0.028 0.002 0.023 20.8 0.27 0.43 0.21 0.065 0.019 0.011 Sn: 0.05, Sb: 0.08 Example 26 0.007 0.09 0.23 0.029 0.002 0.020 20.8 0.18 0.40 0.22 0.074 0.025 0.013 Co: 0.47, Mg: 0.0005 Example 27 0.012 0.08 0.21 0.021 0.002 0.023 21.1 0.22 0.42 0.25 0.085 0.027 0.010 Co: 0.25, V: 0.28, Sn: 0.16 Example 28 0.009 0.08 0.18 0.029 0.002 0.038 21.4 0.15 0.41 0.20 0.099 0.036 0.011 Mo: 0.28, B: 0.0018, Ca: 0.0023 Example 29 0.008 0.11 0.16 0.024 0.002 0.035 21.0 0.27 0.42 0.26 0.071 0.033 0.010 W: 0.45, Mg: 0.0025, Sb: 0.04 Example 30 0.013 0.12 0.22 0.024 0.002 0.021 20.9 0.24 0.41 0.18 0.088 0.031 0.009 Co: 0.04, Y: 0.003, La: 0.007 Example 31 0.008 0.11 0.20 0.026 0.001 0.037 21.3 0.11 0.44 0.24 0.077 0.029 0.011 Mo: 0.17, Ce: 0.05, Sn: 0.001 Example 32 0.008 0.10 0.17 0.026 0.002 0.021 21.3 0.11 0.41 0.21 0.094 0.020 0.013 Sn: 0.005 Example 33 0.009 0.11 0.22 0.021 0.003 0.039 20.8 0.22 0.42 0.24 0.078 0.031 0.010 Sb: 0.13 Example

TABLE 1-2 Test Corrosion Surface Softening No. Ti—Nb Nb—Zr Resistance Charpy Defect Temperature Note 1 0.12 0.044 ◯ ◯ ◯ ◯ Example 2 0.17 0.037 ◯ ◯ ◯ ◯ Example 3 0.10 0.077 ◯ ◯ ◯ ◯ Example 4 0.11 0.077 ◯ ◯ ◯ ◯ Example 5 0.13 0.061 ◯ ◯ ◯ ◯ Example 6 0.13 0.055 ◯ ◯ ◯ ◯ Example 7 0.17 0.046 ◯ ◯ ◯ ◯ Example 8 0.06 0.019 ◯ ◯ ◯ ◯ Example 9 0.15 0.055 ◯ ◯ ◯ ◯ Example 10 0.40 0.056 ◯ ◯ ◯ ◯ Example 11 0.19 0.005 ◯ ◯ ◯ ◯ Example 12 0.13 0.037 ◯ ◯ ◯ ◯ Example 13 0.17 0.115 ◯ ◯ ◯ ◯ Example 14 0.15 0.088 ◯ ◯ ◯ ◯ Example 15 0.15 0.034 ◯ ◯ ◯ ◯ Example 16 0.25 0.002 ◯ ◯ ◯ ◯ Example 17 0.05 0.021 ◯ ◯ ◯ ◯ Example 18 0.08 0.027 ◯ ◯ ◯ ◯ Example 19 0.09 0.033 ◯ ◯ ◯ ◯ Example 20 0.01 0.007 ◯ ◯ ◯ ◯ Example 21 0.05 0.017 ◯ ◯ ◯ ◯ Example 22 0.06 0.029 ◯ ◯ ◯ ◯ Example 23 0.15 0.047 ◯ ◯ ◯ ◯ Example 24 0.12 0.059 ◯ ◯ ◯ ◯ Example 25 0.15 0.046 ◯ ◯ ◯ ◯ Example 26 0.15 0.049 ◯ ◯ ◯ ◯ Example 27 0.17 0.058 ◯ ◯ ◯ ◯ Example 28 0.10 0.063 ◯ ◯ ◯ ◯ Example 29 0.19 0.038 ◯ ◯ ◯ ◯ Example 30 0.09 0.057 ◯ ◯ ◯ ◯ Example 31 0.16 0.048 ◯ ◯ ◯ ◯ Example 32 0.12 0.074 ◯ ◯ ◯ ◯ Example 33 0.16 0.047 ◯ ◯ ◯ ◯ Example *[Corrosion Resistance] A case where the pitting potential was 290 mV (vs. SCE) or more was judged as “◯” (satisfactory), and a case where the pitting potential was less than 290 mV (vs. SCE) was judged as “▴” (unsatisfactory). *[Charpy Impact Value] A case where the Charpy impact value (of a steel sheet having a thickness of 2 mm) at 25° C. was 200 J/cm² or more was judged as “◯” (satisfactory), and a case where the Charpy impact value (of a steel sheet having a thickness of 2 mm) at 25° C. was less than 200 J/cm² was judged as “▴” (unsatisfactory). *[Surface Defect] A case where the number of streaks in a region of 200 mm^(W) × 200 mm^(L) was 1 or less was judged as “◯” (satisfactory), and a case where the above-described number was more than 1 was judged as “▴” (unsatisfactory). *[Softening Temperature] A case where the relational expression c + 0.1 × (a − c) ≥ b was satisfied was judged as “◯” (satisfactory), and other cases were judged as “▴” (unsatisfactory), where the Vickers hardness of a steel sheet in the cold-rolled state was defined as a, the Vickers hardness of a steel sheet which had been subjected to a heat treatment at a temperature of 880° C. for 20 seconds was defined as b, and the Vickers hardness of a steel sheet which had been subjected to a heat treatment at a temperature of 1000° C. for 20 seconds was defined as c.

TABLE 2-1 Chemical Composition (mass %) Other Chemical Test No. C Si Mn P S Al Cr Ni Cu Ti Nb Zr N Element Note 34 0.008 0.10 0.15 0.023 0.002 0.039 19.6 0.29 0.43 0.20 0.090 0.021 0.010 Comparative Example 35 0.012 0.09 0.19 0.025 0.001 0.022 23.2 0.12 0.40 0.20 0.087 0.017 0.009 Comparative Example 36 0.010 0.11 0.18 0.025 0.001 0.034 21.0 — 0.43 0.25 0.063 0.021 0.012 Comparative Example 37 0.008 0.11 0.17 0.022 0.001 0.039 20.8 0.11 0.45 0.09 0.036 0.018 0.013 Comparative Example 38 0.011 0.12 0.19 0.025 0.002 0.030 20.7 0.14 0.44 0.52 0.079 0.018 0.012 Comparative Example 39 0.011 0.11 0.21 0.028 0.002 0.023 20.8 0.28 0.45 0.24 0.008 0.005 0.010 Comparative Example 40 0.007 0.09 0.19 0.025 0.002 0.037 21.4 0.28 0.40 0.42 0.153 0.033 0.013 Comparative Example 41 0.011 0.09 0.21 0.030 0.003 0.030 20.7 0.12 0.43 0.20 0.097 0.003 0.009 Comparative Example 42 0.009 0.11 0.20 0.027 0.003 0.037 20.7 0.24 0.41 0.25 0.071 0.157 0.012 Comparative Example 43 0.008 0.09 0.20 0.025 0.002 0.034 20.9 0.19 0.44 0.13 0.141 0.138 0.010 Comparative Example 44 0.013 0.11 0.21 0.027 0.002 0.035 21.5 0.14 0.45 0.11 0.125 0.094 0.012 Comparative Example 45 0.009 0.11 0.16 0.025 0.002 0.039 21.5 0.13 0.40 0.13 0.147 0.107 0.009 Comparative Example 46 0.008 0.10 0.16 0.024 0.002 0.026 21.1 0.19 0.41 0.10 0.148 0.109 0.008 Comparative Example 47 0.011 0.10 0.16 0.022 0.002 0.039 21.3 0.23 0.42 0.11 0.123 0.018 0.008 Comparative Example 48 0.007 0.11 0.22 0.021 0.001 0.025 21.3 0.22 0.41 0.13 0.148 0.033 0.009 Comparative Example 49 0.009 0.11 0.21 0.030 0.001 0.022 21.4 0.24 0.40 0.11 0.085 0.098 0.010 Comparative Example 50 0.009 0.09 0.15 0.028 0.001 0.026 21.4 0.27 0.44 0.12 0.103 0.132 0.009 Comparative Example 51 0.011 0.12 0.19 0.021 0.002 0.027 20.6 0.25 0.43 0.10 0.064 0.071 0.010 Comparative Example 52 0.012 0.10 0.19 0.023 0.002 0.029 21.0 0.26 0.42 0.13 0.074 0.144 0.009 Comparative Example 53 0.013 0.08 0.17 0.029 0.001 0.021 21.5 0.24 0.43 0.23 0.032 0.051 0.008 Comparative Example 54 0.013 0.11 0.16 0.020 0.002 0.021 20.9 0.29 0.42 0.11 0.051 0.142 0.010 Comparative Example 55 0.010 0.10 0.20 0.020 0.002 0.037 21.4 0.24 0.44 0.12 0.132 0.145 0.008 Comparative Example 56 0.008 0.11 0.20 0.028 0.002 0.024 20.8 0.24 0.43 0.10 0.118 0.133 0.008 Comparative Example 57 0.011 0.10 0.16 0.022 0.001 0.038 21.0 0.23 0.44 0.31 0.001 0.002 0.009 Comparative Example 58 0.009 0.12 0.16 0.022 0.002 0.035 20.6 0.13 0.40 0.02 0.256 0.003 0.011 Comparative Example

TABLE 2-2 Test Corrosion Surface Softening No. Ti—Nb Nb—Zr Resistance Charpy Defect Temperature Note 34 0.11 0.069 ▴ ◯ ◯ ◯ Comparative Example 35 0.11 0.070 ◯ ▴ ◯ ◯ Comparative Example 36 0.19 0.042 ▴ ◯ ◯ ◯ Comparative Example 37 0.05 0.018 ▴ ◯ ◯ ◯ Comparative Example 38 0.44 0.061 ◯ ▴ ▴ ◯ Comparative Example 39 0.23 0.003 ◯ ▴ ▴ ◯ Comparative Example 40 0.27 0.120 ◯ ◯ ◯ ▴ Comparative Example 41 0.10 0.094 ◯ ▴ ▴ ◯ Comparative Example 42 0.18 −0.086 ◯ ▴ ▴ ◯ Comparative Example 43 −0.01 0.003 ◯ ▴ ▴ ◯ Comparative Example 44 −0.02 0.031 ◯ ▴ ▴ ◯ Comparative Example 45 −0.02 0.040 ◯ ▴ ▴ ◯ Comparative Example 46 −0.05 0.039 ◯ ▴ ▴ ◯ Comparative Example 47 −0.01 0.105 ◯ ▴ ▴ ◯ Comparative Example 48 −0.02 0.115 ◯ ▴ ▴ ◯ Comparative Example 49 0.03 −0.013 ◯ ▴ ▴ ◯ Comparative Example 50 0.02 −0.029 ◯ ▴ ▴ ◯ Comparative Example 51 0.04 −0.007 ◯ ▴ ▴ ◯ Comparative Example 52 0.06 −0.070 ◯ ▴ ▴ ◯ Comparative Example 53 0.20 −0.019 ◯ ▴ ▴ ◯ Comparative Example 54 0.06 −0.091 ◯ ▴ ▴ ◯ Comparative Example 55 −0.01 −0.013 ◯ ▴ ▴ ◯ Comparative Example 56 −0.02 −0.015 ◯ ▴ ▴ ◯ Comparative Example 57 0.31 −0.001 ◯ ▴ ▴ ◯ Comparative Example 58 −0.24 0.253 ◯ ▴ ▴ ▴ Comparative Example *[Corrosion Resistance] A case where the pitting potential was 290 mV (vs. SCE) or more was judged as “◯” (satisfactory), and a case where the pitting potential was less than 290 mV (vs. SCE) was judged as “▴” (unsatisfactory). *[Charpy Impact Value] A case where the Charpy impact value (of a steel sheet having a thickness of 2 mm) at 25° C. was 200 J/cm² or more was judged as “◯” (satisfactory), and a case where the Charpy impact value (of a steel sheet having a thickness of 2 mm) at 25° C. was less than 200 J/cm² was judged as “▴” (unsatisfactory). *[Surface Defect] A case where the number of streaks in a region of 200 mm^(W) × 200 mm^(L) was 1 or less was judged as “◯” (satisfactory), and a case where the above-described number was more than 1 was judged as “▴” (unsatisfactory). *[Softening Temperature] A case where the relational expression c + 0.1 × (a − c) ≥ b was satisfied was judged as “◯” (satisfactory), and other cases were judged as “▴” (unsatisfactory), where the Vickers hardness of a steel sheet in the cold-rolled state was defined as a, the Vickers hardness of a steel sheet which had been subjected to a heat treatment at a temperature of 880° C. for 20 seconds was defined as b, and the Vickers hardness of a steel sheet which had been subjected to a heat treatment at a temperature of 1000° C. for 20 seconds was defined as c.

TABLE 3-1 Chemical Composition (mass %) Other Chemical Test No. C Si Mn P S Al Cr Ni Cu Ti Nb Zr N Element Note 59 0.009 0.08 0.16 0.027 0.002 0.029 20.6 0.28 0.32 0.13 0.118 0.038 0.013 Mo: 0.09 Example 60 0.009 0.10 0.17 0.026 0.003 0.029 21.4 0.30 0.33 0.15 0.139 0.022 0.010 V: 0.11 Example 61 0.012 0.11 0.21 0.027 0.002 0.024 21.1 0.21 0.44 0.16 0.146 0.029 0.009 Mo: 0.26 Example 62 0.010 0.11 0.23 0.026 0.003 0.031 20.9 0.23 0.41 0.22 0.056 0.050 0.010 Ca: 0.0024 Example 63 0.008 0.12 0.20 0.029 0.001 0.021 21.0 0.14 0.44 0.26 0.091 0.082 0.011 W: 0.33 Example 64 0.012 0.10 0.18 0.025 0.003 0.038 21.3 0.22 0.43 0.25 0.113 0.105 0.011 B: 0.0015 Example 65 0.009 0.11 0.23 0.026 0.003 0.037 20.7 0.19 0.57 0.22 0.126 0.102 0.012 Co: 0.18 Example 66 0.010 0.12 0.23 0.025 0.001 0.022 20.9 0.22 0.78 0.23 0.128 0.119 0.010 La: 0.08 Example 67 0.009 0.09 0.22 0.022 0.002 0.020 21.2 0.27 0.41 0.25 0.116 0.120 0.007 Comparative Example 68 0.008 0.10 0.15 0.029 0.002 0.023 21.5 0.20 0.45 0.21 0.129 0.141 0.013 Comparative Example

TABLE 3-2 Test Corrosion Surface Softening No. Ti—Nb Nb—Zr Resistance Charpy Defect Temperature Note 59 0.01 0.080 ◯ ◯ ◯ ◯ Example 60 0.01 0.117 ◯ ◯ ◯ ◯ Example 61 0.01 0.117 ◯ ◯ ◯ ◯ Example 62 0.16 0.006 ◯ ◯ ◯ ◯ Example 63 0.17 0.009 ◯ ◯ ◯ ◯ Example 64 0.14 0.008 ◯ ◯ ◯ ◯ Example 65 0.09 0.024 ◯ ◯ ◯ ◯ Example 66 0.10 0.009 ◯ ◯ ◯ ◯ Example 67 0.13 −0.004 ◯ ▴ ▴ ◯ Comparative Example 68 0.08 −0.012 ◯ ▴ ▴ ◯ Comparative Example *[Corrosion Resistance] A case where the pitting potential was 290 mV (vs. SCE) or more was judged as “◯” (satisfactory), and a case where the pitting potential was less than 290 mV (vs. SCE) was judged as “▴” (unsatisfactory). *[Charpy Impact Value] A case where the Charpy impact value (of a steel sheet having a thickness of 2 mm) at 25° C. was 200 J/cm² or more was judged as “◯” (satisfactory), and a case where the Charpy impact value (of a steel sheet having a thickness of 2 mm) at 25° C. was less than 200 J/cm² was judged as “▴” (unsatisfactory). *[Surface Defect] A case where the number of streaks in a region of 200 mm^(W) × 200 mm^(L) was 1 or less was judged as “◯” (satisfactory), and a case where the above-described number was more than 1 was judged as “▴” (unsatisfactory). *[Softening Temperature] A case where the relational expression c + 0.1 × (a − c) ≥ b was satisfied was judged as “◯” (satisfactory), and other cases were judged as “▴” (unsatisfactory), where the Vickers hardness of a steel sheet in the cold-rolled state was defined as a, the Vickers hardness of a steel sheet which had been subjected to a heat treatment at a temperature of 880° C. for 20 seconds was defined as b, and the Vickers hardness of a steel sheet which had been subjected to a heat treatment at a temperature of 1000° C. for 20 seconds was defined as c.

FIG. 1 illustrates the evaluation results of Charpy impact values and surface defects of the examples of the present invention and comparative examples (Nos. 43 through 48), whose chemical compositions were within the range according to aspects of the present invention, and in which the relational expression Nb Zr was satisfied and the relational expression Ti Nb was not satisfied, in a form of graph in which the horizontal axis indicates the Ti content and the vertical axis indicates the Nb content. Here, for all the steel sheets illustrated in FIG. 1, in a case where the evaluation result regarding a Charpy impact value was satisfactory, the evaluation regarding a surface defect was satisfactory, and in a case where the evaluation result regarding a Charpy impact value was unsatisfactory, the evaluation regarding a surface defect was unsatisfactory. As FIG. 1 indicates, it is necessary that the relational expression Ti≥Nb be satisfied in order to realize excellent toughness and a decrease in the quantity of surface defects at the same time within the range of the chemical composition according to aspects of the present invention.

FIG. 2 illustrates the evaluation results of Charpy impact values and surface defects of the examples of the present invention and comparative examples (Nos. 49 through 54, 67, and 68), whose chemical compositions were within the range according to aspects of the present invention, and in which the relational expression Ti Nb was satisfied and the relational expression Nb Zr was not satisfied, in a form of graph in which the horizontal axis indicates the Nb content and the vertical axis indicates the Zr content. As FIG. 2 indicates, it is necessary that the relational expression Nb≥Zr be satisfied in order to realize excellent toughness and a decrease in the quantity of surface defects at the same time within the range of the chemical composition according to aspects of the present invention. Moreover, as FIG. 1 and FIG. 2 indicate, it is clarified that it is necessary that both the relational expression Ti≥Nb and the relational expression Nb≥Zr, that is, the relational expression Zr≤Nb≤Ti, be satisfied in order to realize excellent toughness and a decrease in the quantity of surface defects at the same time within the range of the chemical composition according to aspects of the present invention.

Here, comparative example Nos. 55 and 56, whose chemical compositions were within the range according to aspects of the present invention, and in which the relational expression Ti≥Nb. or the relational expression Nb≥Zr was not satisfied, were unsatisfactory in the evaluations both regarding a Charpy impact value and regarding a surface defect.

INDUSTRIAL APPLICABILITY

The ferritic stainless steel sheet according to aspects of the present invention, which has excellent toughness and only a small quantity of surface defects, can preferably be used not only for parts which are required to have satisfactory corrosion resistance but also for parts which are required to have satisfactory toughness and surface quality including the inner panels of elevators, interiors, duct hoods, muffler cutters, lockers, the parts of home electrical appliances, the parts of office appliances, the interior parts of automobiles, the exhaust pipes of automobiles, building materials, the lids of drainage channel, maritime containers, house wares, kitchen appliances, the interior and exterior materials of buildings, automobile parts, escalators, railway vehicles, the chassis of electrical apparatuses and the like. 

1. A ferritic stainless steel sheet having a chemical composition containing, by mass %, C: 0.020% or less, Si: 0.05% to 0.40%, Mn: 0.05% to 1.00%, P: 0.040% or less, S: 0.030% or less, Al: 0.001% to 0.15%, Cr: 20.0% to 23.0%, Ni: 0.01% to 0.80%, Cu: 0.30% to 0.80%, Ti: 0.10% to 0.50%, Nb: 0.010% to 0.150%, Zr: 0.005% to 0.150%, N: 0.020% or less, and the balance being Fe and inevitable impurities, in which relational expression (1) below is satisfied: Zr≤Nb≤Ti  (1), where each of Zr, Nb, and Ti in relational expression (1) denotes the content (mass %) of the corresponding chemical element.
 2. The ferritic stainless steel sheet according to claim 1, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from Co: 0.01% to 0.50%, Mo: 0.01% to 0.30%, and W: 0.01% to 0.50%.
 3. The ferritic stainless steel sheet according to claim 1, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from V: 0.01% to 0.50%, B: 0.0003% to 0.0030%, Mg: 0.0005% to 0.0100%, Ca: 0.0003% to 0.0030%, Y: 0.001% to 0.20%, and REM (rare-earth metal): 0.001% to 0.10%.
 4. The ferritic stainless steel sheet according to claim 2, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from V: 0.01% to 0.50%, B: 0.0003% to 0.0030%, Mg: 0.0005% to 0.0100%, Ca: 0.0003% to 0.0030%, Y: 0.001% to 0.20%, and REM (rare-earth metal); 0.001% to 0.10%.
 5. The ferritic stainless steel sheet according to claim 1, the steel sheet having the chemical composition further containing, by mass %, one or both selected from Sn: 0.001% to 0.50% and Sb: 0.001% to 0.50%.
 6. The ferritic stainless steel sheet according to claim 2, the steel sheet having the chemical composition further containing, by mass %, one or both selected from Sn: 0.001% to 0.50% and Sb: 0.001% to 0.50%.
 7. The ferritic stainless steel sheet according to claim 3, the steel sheet having the chemical composition further containing, by mass %, one or both selected from Sn: 0.001% to 0.50% and Sb: 0.001% to 0.50%.
 8. The ferritic stainless steel sheet according to claim 4, the steel sheet having the chemical composition further containing, by mass %, one or both selected from Sn: 0.001% to 0.50% and Sb: 0.001% to 0.50%. 